Your body starts acting weird

The human body has an internal temperature of around 37oC, and it does NOT like it when that very specific figure wobbles in either direction.

Changes of as little as a single degree can cause your body’s delicate biochemistry to glitch in unpleasant ways.

WATCH: We put a reporter in 50°C hot car. See what happens

You’d think that prolonged exposure to heat and humidity would have straightforward effects (you’ll read more about heat exhaustion and heat stroke down below), but sometimes the resulting malfunctions can take some odd forms.

Ever had a muscle cramp on a really hot day, say, while doing some heavy lifting? There’s a good chance the temperature is to blame.

What’s happened is that you’ve sweated out a lot of water, but even if you’ve been drinking water to replace it, you’re not getting enough electrolytes. The resulting salt imbalance is what’s causing the cramps.

For people really not used to the heat, there’s a risk of heat edema. To avoid overheating, your body dilates your blood vessels to try radiate as much heat away from your system as possible, causing blood to pool in your ankles.

Even sweating can sometimes not go as it’s supposed to. If you develop tiny red spots on your skin, with a prickly sensation, that’s a heat rash, caused when your sweat pores become blocked.

And according to the U.S. Centres for Disease Control, if you are exposed to extreme heat for a prolonged period, you may stop sweating altogether, a step on the road to potentially fatal heat stroke.

Before you get there, however, the extreme heat will do some unpleasant things to your head as well.

Your brain doesn't work right

In 2014, Canadian tennis pro Frank Dancevic had a rather unusual court invasion.

Like the other players at the Australian Open in Melbourne, he was struggling through a set in record heat, which became so overwhelming he apparently started seeing comic book characters.

"I was dizzy from the middle of the first set and then I saw Snoopy and I thought, 'Wow Snoopy, that’s weird,'" Dancevic said, according to Slate Magazine. "I couldn’t keep my balance anymore and I leaned over the fence and when I woke up people were all around me."

Confusion and dizziness are common effects of too much exposure to extreme heat, thanks to increased blood flow to dilated blood vessels and fluid loss through sweating.

It’s a real hazard for workers who need to keep their concentration in such conditions. The Canadian Centre for Occupational Health and Safety (CCOHS) says aside from increased irritability, you start losing the ability to do skilled or mental tasks.

As for Dancevic’s loss of consciousness, that was potentially something called heat syncope, a temporary drop in blood flow to the brain which occurs when you’ve lost a lot of body fluid due to sweating and low blood pressure.

That could also have accounted for the dizziness too, but given his prolonged exposure, it was just as likely heat exhaustion.

Heat exhaustion sets in

So far we’ve talked about the strange little symptoms of excessive heat, like heat rashes and blood pooling in the ankles. Heat exhaustion is when you’ve lost enough body fluids and salt that the body starts losing its ability to cope.

Your nausea may reach the point you start to vomit, and diarrhea may set in. Those muscle cramps we talked about earlier may happen more frequently, you’ll experience palpitations, and tingling or numbness of the hands or feet.

You may not feel all of these at once, but even a couple of these symptoms in either yourself or a companion should be enough to raise the alarm.

And you’d better do it fast. Without treatment, the sufferer may find themselves in the grip of potentially deadly heat stroke.

Heat stroke takes you to death's door

If you think you’re seeing signs of full-fledged heat stroke, all medical authorities say to call 911. It is that serious.

If someone is at the stage of heat stroke, their core temperature is at 40oC or greater, and their body’s mechanisms for handling the heat have failed. That can include the loss of so much body fluid that sweating stops altogether (although that does not occur in all cases).

Heat stroke shares some symptoms with heat exhaustion, but the most acute to watch out for (according to CCOHS) are:

Hot, dry skin or profuse sweating

Confusion

Loss of consciousness.

Seizures

Very high body temperature

It can follow on from untreated heat exhaustion, or can happen without little or no warning.

While you’re waiting for medical help to arrive, the victim has to be moved to a cool, shaded place and have as many clothes removed as possible. Cold cloths or ice should be applied, but CCOHS says not to try force the person to drink liquids.

And all this has to be done fast. Although prompt treatment can guarantee survival, the American Family Physician says heat stroke can have a mortality rate of 10 per cent.

Yes, people absolutely can die in extreme heat events. And quite a few have.

Heat waves can kill thousands

When officials decide whether to issue a heat warning, Europe's summer of 2003 can't be far from their thinking.

In August of that year, a stubborn area of high pressure parked itself over a large chunk of the continent, sending temperatures soaring to record levels for weeks at a time.

The result: An estimated 70,000 people lost their lives, with 15,000 in France.

Cities in heat warnings urge people to check on the elderly, and they were the ones who suffered the most in Europe. Live Science says many of the dead were elderly women living in the upper floors of poorly ventilated apartment buildings. Heat would have risen via convection, making it hotter and hotter.

In the United States, more than 600 people die every year due to extreme heat, according to the Centres for Disease Control. In the 1995 Chicago heat wave, almost 700 people lost their lives, and thousands went to the emergency room.

It can be hard to get exact numbers however, as often the bodies are discovered after the extreme heat has subsided, making it difficult to ascribe temperature as the main cause of death.

Still, even though the numbers may be fuzzy, the effects of extreme heat aren't: If you don't take it seriously, it can kill.

Keep that in mind the next time you feel like going for a run in 35-degree weather.